1. Field of the Invention
This invention pertains to electrically conductive vitreous enamel compositions and methods in general, and particularly to low Temperature Coefficient of Resistance (TCR) nickel chromium compositions and methods of manufacture thereof.
2. Description of the Related Art
In recent years, a need has developed for low sheet resistance and low TCR thick film resistors for use in high power and lightning surge protection applications. These special resistors should have a sheet resistance from 1/4 to a few ohms per square with a TCR less that .+-.100 ppm/.degree.C. The characteristics and performance of these resistors should be comparable to those of the best ruthenium-based resistors.
TCR stands for Temperature Coefficient of Resistance, which is a measure of the amount of change in resistance over some temperature range. For the purposes of the remainder of this disclosure, TCR may be further divided into cold TCR (CTCR) and hot TCR (HTCR). Cold TCR is measured over the temperature range from -55 to +25 degrees Centigrade, while hot TCR is measured from +25 to +125 degrees Centigrade.
Resistivity for the purposes of this disclosure is measured in the units of ohms per square. This will be considered herein to be the resistance of a 1 mil thick film of equal length and width.
Resistors made from palladium-silver compositions have been used for low sheet resistance and low TCR applications. However, the cost of precious metals, especially with a high percentage of palladium, is much higher than the cost of base metals. In addition, the palladium-silver circuits may be plagued by silver migration problems.
Nichrome has been produced in wire form for heating elements for more than a century. The nickel chromium alloy is very stable, as demonstrated by the long durations of operation at very elevated temperature. In fact, nickel chromium alloys are the lowest cost and most commonly used heating alloys for temperatures up to approximately 1,000.degree. C. Wirewound resistors have been similarly formed from Nichrome, to attempt to take advantage of these beneficial characteristics. However, these wire resistors exhibit inductance problems and are of large size, thereby limiting application.
Nichrome films formed by vacuum deposition have been used in thin film circuits for many years. The thick film approach is easier and more cost effective than thin film, thus limiting the use of thin film to special applications.
Thick film ink, using pre-alloyed nichrome powder, can meet many of the specifications required for surge and power applications for relatively low cost. However, an ink manufacturer is limited to the selection of commercially available alloys. To form alloys and then to mill the alloys into sub-micron sizes adds to the cost of materials and prolongs delivery time. Impurities during the milling process may also introduce manufacturing problems and affect quality and performance of the resulting resistor.